Subatomic Particles and the Nuclear Atom Section 4.2 Subatomic Particles and the Nuclear Atom

Discovery of the Atom and its Particles The men whose quests for knowledge about the fundamental nature of the universe helped define our views.

Dalton’s Model In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

Dalton’s Theory He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles. Atoms of the same element are exactly alike. Atoms of different elements are different. Compounds are formed by the joining of atoms of two or more elements.

. This theory became one of the foundations of modern chemistry.

Cathode Ray Accidental discovery of William Crookes He noticed a flash of light within the tubes of a vacuum pump Ray of radiation originating from the cathode end of the tube Stream of negatively charged particles Led to the development of the television

Thomson’s Plum Pudding Model In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

Thomson Model He proposed a model of the atom that is sometimes called the “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.

Thomson Model Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles.

Thomson Model This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from? Where did they come from?

Thomson concluded that the negative charges came from within the atom. A particle smaller than an atom had to exist. The atom was divisible! Thomson called the negatively charged “corpuscles,” today known as electrons. Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom. But he could never find them.

Rutherford’s Gold Foil Experiment In 1908, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure.

Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all. Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.” He called the center of the atom the “nucleus” The nucleus is tiny compared to the atom as a whole.

Rutherford Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.

Subatomic Particles Proton- positively charged particles found in the nucleus Neutron- particles with no charge also found in the nucleus Electron- negatively charged particles that orbit the nucleus and make up the volume of the atom

Particle Charge Mass (amu) Proton Positive (+1) 1.0073 Neutron None (neutral) 1.0087 Electron Negative (-1) 5.486 x 10-4